1. Field of the Invention
The present invention relates to a solar generation system. More specifically, the present invention relates to a solar generation system in which a DC power generated by an independent DC power source such as a solar cell is boosted by a booster unit and converted to an AC voltage by an inverter apparatus to supply power to general AC load for home and office use, or to feed power to existing utility power supply.
2. Description of the Background Art.
A solar cell as a DC power source outputs a DC power when there is high solar insolation. The DC power can be output solely by the solar cell without using other energy source such as a storage battery, and no poisonous substance is discharged. Therefore, the solar cell has been known as a simple and clean energy source.
FIG. 15 is a block diagram showing an example of a conventional solar generation system. For simplicity of the drawing, only two solar cell strings 1a and 1b are shown in the solar generation system. It is needless to say that normally, a larger number of solar cell strings are provided. Generally, one standard solar cell string includes eight or nine solar cell modules (not shown) connected in series with each other.
In the solar generation system, when the DC output power from solar cell strings 1a and 1b is converted to an AC power and interconnected to a utility power supply 4, it is necessary to interpose a power conditioner 3 between the solar cell strings 1a, 1b and the utility power supply 4. When a plurality of solar cell strings 1 are to be interconnected to the utility power supply 4, the plurality of solar cell strings 1 are connected in parallel with the power conditioner 3. Power conditioner 3 includes backflow preventing diodes 50a and 50b, so as to prevent backflow of the current generated by the plurality of solar cell strings 1 connected in parallel. The DC power that has passed through backflow preventing diodes 50a and 50b is converted to an AC power by a DC/AC inverter 60, and supplied through a protection circuit 70 to the utility power supply 4.
Conventionally, it is a common practice for the solar generation system in Japan that a plurality of solar cell strings included therein are placed on a main portion of a roof facing southward, and lines from the solar cell strings are connected to power conditioner 3.
When the solar cell strings are to be placed on the roof of a house, sometimes it is difficult to configure solar cell strings by arranging solar cell modules only that surface of the roof which faces southward and receiving the most of the sunshine. Solar cell modules that are positioned not on the southward surface of the roof may be arranged on the eastward or westward surface of the roof to form the solar cell strings. Sometimes, the solar cell strings are configured by placing small size solar cell modules arranged in the remaining peripheral regions after the solar cell modules are placed on the main portion of the southward surface of the roof. More specifically, sometimes the number of series-connected solar cell modules included in some solar cell strings is different from other solar cell strings. In such a case, different output voltages result from different solar cell strings.
For example, when a standard solar cell string including the standard number of series-connected solar cell modules and a substandard solar cell string including series-connected modules of smaller than the standard number are connected in parallel to the power conditioner 3, only the power from the standard solar cell having the standard output voltage is input to power conditioner 3, and the power from the substandard solar cell string having the substandard output voltage lower than the standard output voltage cannot be fed to the power conditioner 3. Even when the power from the substandard solar cell string is adapted to be fed to power conditioner 3, it is impossible to obtain the maximum output power that is the sum of the maximum power from the standard solar cell string and the maximum power from the substandard solar cell string, as can be seen from FIGS. 16A and 16B.
Unless the power from such a substandard solar cell string can be fed efficiently to power conditioner 3, the area occupied by the substandard solar cell string would be wasted.
In the graphs of FIGS. 16A and 16B, the abscissa represents output voltage V and the ordinate represents output power P. In the graph of FIG. 16A, the curve S represents an output power from the standard solar cell string, while the curve N represents the output power from the substandard solar cell string. More specifically, the standard solar cell string has the maximum output power Ps, while the substandard solar cell string has the maximum output power Pn. The output power that is the sum of these two output powers is as shown in FIG. 16B. The maximum output power Psn of the output power curve (S+N) shown in FIG. 16B is considerably smaller than the sum (Ps+Pn) of the maximum output powers Ps and Pn shown in FIG. 16A. The reason for this is that the voltage position for the maximum output power Ps of the standard solar cell string 1a is different from the voltage position of the maximum output power Pn from the substandard solar cell string 1b. 
In view of the foregoing, a possible solution is to adjust output voltages from the plurality of solar cell strings. For this purpose, an impedance may be interposed between standard solar cell string 1a and power conditioner 3. This method, however, is not practical, as the power is lost by the impedance. Another possibility is to use MG (Motor Generator) method to change the DC voltage. This method, however, is not preferable as mechanical vibration or noise is generated and the motor generator itself is bulky.
In the solar cell generation system disclosed in Japanese Patent Laying-Open No. 8-46231, boosted type DC-DC converter 80b having maximum power point tracking function are incorporated in each solar cell module or in each substandard solar cell string, as shown in FIG. 17. Such a solar generation system is disadvantageous in that the circuit structure becomes complicated and in that voltage adjustment for the solar generation system as a whole must be performed in the initial design stage of each solar cell string having different output voltages.
In Japanese Patent Laying-Open No. 8-46231, an isolation transformer is connected. This increases the weight of the system and lowers power conversion efficiency. In case of a malfunction of the boosting circuit caused by a surge, it will trouble a repair person to climb on the roof and to exchange the solar cell module.
Therefore, a main object of the present invention is to enable interconnection of a plurality of solar cell strings having different output voltages to a utility power supply in a simple manner, and to enable efficient use of the maximum output power of the solar cell strings.
Briefly stated, the present invention relates to a solar generation system in which a DC voltage output from a solar cell is boosted, and the boosted DC voltage is supplied to an inverter apparatus converting the DC voltage to an AC power, including a standard solar cell string having a standard number of solar cell modules connected in series, a substandard solar cell string having solar cell modules smaller in number than the standard number connected in series, a boosting circuit for boosting the DC voltage output from the substandard solar cell string to a DC voltage output from the standard solar cell string, and an input connecting circuit for supplying the DC voltage boosted by the boosting circuit and the DC voltage output from the standard solar cell string to the inverter apparatus.
Therefore, according to the present invention, as the DC voltage from the substandard solar cell string is increased to the DC voltage of the standard solar cell string, interconnection to the utility power supply is possible in a simple manner, and the sum of the maximum outputs from respective solar cell strings can be used as the final maximum output power.
More preferably, the boosting circuit boosts the DC voltage output from the substandard solar cell string at a boosting voltage ratio determined by the ratio between the standard number and the number smaller than the standard number.
More preferably, the system includes a switch for manually switching the boosting voltage ratio of the boosting circuit.
More preferably, the system includes a control circuit for controlling the boosting circuit by setting the boosting voltage ratio by pulse width modulation.
More preferably, a plurality of substandard solar cell strings are provided, and boosting circuits are provided for respective ones of the plurality of substandard solar cell strings, for boosting the DC voltage output from the corresponding one of the substandard solar cell strings.
More preferably, the boosting circuit is provided detachably between the substandard solar cell strings and the input connecting circuit.
More preferably, a power supply voltage is supplied to the boosting circuit from the substandard solar cell string.
More preferably, the input connecting circuit includes a backflow preventing circuit for preventing backflow of the current from the side of the boosting circuit to the substandard solar cell string, an input connecting/disconnecting circuit for connecting or disconnecting the substandard solar cell string and the boosting circuit, and a lightning surge preventing circuit for preventing entrance of lightning surge from the substandard solar cell string to the side of the boosting circuit.
More preferably, the system includes a voltage control circuit performing control to keep constant the boosting ratio, when the output voltage of the boosting circuit is lower than an upper limit set voltage.
More preferably, when the output voltage of the boosting circuit is higher than the upper limit set voltage, the voltage control circuit performs control to keep constant the upper limit voltage.
More preferably, the voltage control circuit changes the boosting ratio.
More preferably, the input connecting circuit includes a trip signal generating circuit generating a trip signal when the output voltage attains to an over voltage, and a breaker opening the connection between the substandard solar cell string and the input connecting circuit in response to the trip signal from the trip signal generating circuit.
More preferably, the trip signal generating circuit opens connection between the substandard solar cell string and the input connecting circuit by means of the breaker, by generating the trip signal, when there is a short-circuit in the boosting circuit.
More preferably, the trip signal generating circuit outputs a trip signal when it is detected that a short-circuit current flows in the boosting circuit and the temperature is increased.
More preferably, the trip signal generating circuit generates the trip signal when the output voltage of the boosting circuit exceeds a predetermined input voltage range.
More preferably, the boosting circuit includes a fuse for intercepting the short-circuit current from an output side.
More preferably, the fuse is connected in series with the boosting circuit, and opens the path of the short-circuit current, in accordance with the magnitude of the short-circuit current.
More preferably, the system includes a box placed outdoors, housing at least the input connecting circuit, and the box includes a drainage to guide rain water to a lower portion when rain water enters, and an outlet opening for discharging the rain water guided to the lower portion.
More preferably, a radiator is provided outside the box, for generation of heat from the boosting circuit and the backflow preventing circuit.
More preferably, the system includes a metal plate covering the radiator of the box and supporting the box on a wall surface.
More preferably, the box has a lid that can be opened/closed, and the input connecting circuit is operated with the lid opened.
More preferably, the system includes an indicator which is turned on when the boosting circuit is driven, and which is turned off in response to the stop of operation of the boosting circuit.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.